Traveling wave tubes



June 9, 1959 Filed July 9, 1956 3 Sheets-Sheet 1 FIG 1 'June 9, .1959 K D CHARLES ETAL 2,890,374

TRAVELING WAVE TUBES Filed July 9, 1956 3 Sheets-Sheet 2 FIG..3

June 9, 1959 Filed July 9, 195e D. CHARLES ETAL TRAVELING WAVE TUBES 3 Sheets-Sheet 3 nited States atent mi TRAVELING WAVE TUBES Daniel Charles and Roger Gentner, Paris, France, as-

signors to Compagnie Generale de Telegraphie Saus Fil, a corporation of France The present invention relates to traveling wave tubes with no transversal magnetic field and more part1cularly to such tubes in which the electron beam is of rectangular vcross section.

' The` basic principle of traveling wave tubes is well known and the backward travel-ing wave oscillator has been disclosed in the copending patent application Serial No.l 281,347, filed April 9, 1951, by Bernard Epsztein. This tube comprises a delay circuit, the ends of which are mutually yuncoupled, one end having a cathode adjacent thereto being coupled to the output and the opposite end being provided with an absorbing means. As disclosed in the above eopending application, the operation of this tube is based on the principle of interaction .between the electron beam and a backward space harmonic of the wave propagating in the delay circuit.

In traveling wave tubes of the type considered, focusing'v-Qf vthe electron beam allV along its pat-h is` generally effected by means f a magnetic fieldI Perma-nent mas nets or electromagnets are provided to this effect. This, however, considerably `increases the size and the weight off the tube, which results in an increase of `the cost price thereaf- It is an object ofk the present invention to eliminate these drawbacks.

The; tube according tothe invention makes use of an electron beam having a rectangular cross section, and comprises an interdigital delay line consisting of two combs, brought to different potentials, the comb fingers having a closed contour surrounding the rectangular bam0nbeing of the So-.called hairpin type.

Acszording; to,- the invention, the two arms of each hairpin'are joined by symmetrical portions, defining a plurality of parallelly arranged contours which form a passage or channel, for the propagation of the beam, and anV additional, plane electrode is located, along each comb, in parallel relationship with the inner faces thereof, these two plane electrodes being brought to difierent potentials. i i' Preferably, two further electrodes are placed on both sides of the `delay line and parallel thereto, the -two electrodes being brought to a potential near that of the sfhode The'invention will be better understood from the ensuing #description iny connection with the appended drawings illustrating, by way of example, a preferred embodiment of lthe tube. according to the invention and wltreinr Fig. 1 diagrammatically shows a perspective view as seen.V from beneath, of a tube according lto the invention;

Eig. 2 is an axial section of an embodiment of the tube ofFig, 1;

Fig. 3 is a cross-section along line III-III of Fig. 2;

Fig. 4 is a modification of the finger of the delay-line in` the, tube of they invention;

Patented June 9, 1959 Figs. 5 and 6 show a method for mounting the fingers on the bars of the delay line.

Similar reference numerals designate similar elements throughout all the figures.

As shown in Fig. 1, the tube comprises a cylindrical, vacuum-tight envelope 1, containing an electron gun comprising, for example, a cathode 2, indirectly heated by means of a wire 3, a focusing electrode 6, having a rectangular opening 7, located in front of the emissive face of cathode 2, and an accelerating anode 8, also provided with a rectangular opening 9, placed in front of opening 7. Connections` 4, 5, 10 and 11 are .provided for bringing cathode 2, wire 3, focusing electrode 6 and accelerating anode 8 to suitable potentials.

The tube comprises a delay line A, consisting of two identical combs B. Each comb comprises a bar 12, or 13, and an angle bar 22, or 20, is positioned along the bar with one flange extending along the inner side of the bar at a certain distance therefrom and parallel thereto, while the other flange is fastened to the upper face of the bar and is insulated therefrom.

A row of equally spaced windows 21 -is provided in the respective flanges of the angle bars parallel to the inner faces of the bars 12 and 13. A hairpin shaped finger 14, fastened by its `free end to the bar, extends through each window 21. The two legs of each hairpin are joined, at a certain distance from the bar to which it is fastened, by a rounded portion 15, symmetrical with the rounded end portion 16 of the hairpin. Thus, each hairpin comprises a closedcontour C.

The two combs B are parallel, each finger of one comb being located between two fingers of the other, i.e. the fingers are interdigitated with all the contours C parallel to form a channel.

The cathode 2 is at one end of the delay line and a collector 26 at the other end, so as to cause the emitted beam to propagate in the channel formed by contours C.

According to another feature of the invention, two grid shaped electrodes 24 and 25 extend on each side of the idelay line. They are ih parallel relationship to the line and are insulated therefrom. Connections 27, 28, 29, 3G, 31,y 32, 33 are arranged for bringing the combs 12, 13, the angle bars 20, 22, the electrodes 24 and 25, and the collector electrode 2 6, to the required potentials as described hereinafter. A source 34 supplies` the heating current for the cathode and a battery 35 supplies the different potentials required.

Since the tube illustrated, by way of example, in Fig. 1, is a backward wave traveling wave oscillator, the end of lthe delay line adjacent collector 26 is provided with absorbing means, which in the embodiment shown is constituted by an absorbing coating on fingers 18 and 19. Further the output 17 is coupled to the finger 60 adjacent cathode 2, all as, described in the above mentioned patent application.

Taking the potential to which the cathode 2 is raised as the reference potential, focusing electrode 6 is brought to a negative potential, and the accelerating anode 8 to a positive potential, for instance 400 v., by an adjustable tap 61.

Comb 13 carrying finger 60 is brought to a higher potential Ithan the accelerating anode 8, for instance 800 v. Comb 12 may be brought to a potentialA com-` prised .between that of the cathode 2 and that of the accelerating anode 8, for instance between 0 and 100` v.,

by means of an adjustable tap 36. Angle bar 22 and` 12 and 13 are each closer to the potential of the respective remote bar than to that of the proximate bar and are maintained at the same or approximately the same potential as the remote bar and the fingers connected thereto.

The operation of the tube is as follows:

Cathode 2, which is heated by helical filament 3, emits electrons throughout its emissive surface. These electrons are focused by electrode 6 and successively accelerated by accelerating anode 8 and finger 60. The cross section of the electron beam, when it reaches the level of finger 60, is rectangular.

It is known that the edges of a flat beam tend, due to the space charge, to diverge from each other and trace parabolae. The unit formed by the two first fingers of the delay line, which are brought to different potentials, and by the opposite faces of the two angle bars 20 and 22, also brought to different potentials, is equivalent to a cylindrical, converging electrostatic lens which enables the beam substantially to maintain its initial cross section.

Each successive pair of fingers has a similar effect and acts like a converging electrostatic lens which opposes the natural tendency of the beam to diverge. Applicants have established, experimentally, that the better the characteristics of geometrical symmetry of the delay line considered as a whole, the more effective the `focusing effect.

According to the invention, this effect is further enhanced by the elements and the angle bars 20 and 22.

If comb 13 is brought to a potential V13 and angle bar 20, adjacent thereto, to a potential V20, Comb 12 to a potential V12 and angle bar 22, adjacent thereto, to a potential V22, experience shows that, with V13 higher than V12, the best focusing effect is obtained with V lower than V22.

In order to obtain best operating conditions, the beam should pass very close to the metal rods forming the fingers. However, in this case, a substantial number of electrons will strike the fingers which, being usually of copper, will become the source of a strong secondary emission. Secondary electrons emitted from these fingers are accelerated and bombard the envelope of the tube. All this results in energy loss in the case of a metal envelope or in parasitic charges in the case of an insulating envelope.

Electrodes 24 and 25 which are at the potential of the cathode 2 prevent the secondary electrons lfrom reaching the tube envelope.

Figs. 2 and 3 show in more detail an embodiment of the tube according to the invention. Envelope 1, for instance of copper, carries a metal base 38 brazed thereto. A plane metal plate 41, carrying the various tube elements, is fastened with screws 39 and 40 to base 38. Collector 26 is attached by screws 42 to plate 41. At the other end of this plate 41, cathode 2 is rigidly fastened to focusing electrode 6, with interposition of insulating elements. The unit formed by focusing electrode 6 and accelerating anode 8 is solidly maintained in position by means of a strap 59, shown in section in Fig. 2 and rigidly secured by both its ends to plate 41. Bars 12 and 13 of the delay line, angle bars 20, 22 and electrodes 24, are assembled by means of two pairs 43 and 58 of threaded rods and nuts 45 and 46, with interposition of insulating pieces 44, for instance of alumina, or some ceramic material to keep these various members insulated from each other. Bars 12 and 13, carrying the delay line fingers, are provided with openings 47 and `48, having the shape of button-holes, to allow for the expansion under the action of temperature variations, and rods 43 extend through these openings. Moreover, a channel 49 is cut in the outer half of bar 13 throughout its length, to allow for the passage of an output coaxial cable 50,

the central conductor of which is brazed to the finger adjacent to the cathode 2.

Applicants have built a tube, similar to that represented in Figs. l, 2 and 3, capable of delivering oscillations in a continuous frequency band between 18 and 38 cm. This result has been obtained by maintaining the accelerating anode voltage constant at 400 v., by varying in a wide range, for instance between and 1200 v., the voltage applied to comb 13 and by suitably adjusting each time the voltage of comb 12.

Fig. 4 shows a modification of the delay line, used in the tube according to the invention. The rounded portions 15 and 16 of the fingers in Fig. 3 are substituted by straight portions 51 and 52. The channel through which the rectangular beam propagates in the delay line has in this case also a perfectly rectangular cross section, which appears to improve coupling between the beam and the field in the line.

Figs. 5 and 6 show the two main construction stages of a delay line of the type represented in Figs. l, 2 and 3. In a first stage, bars 53 are provided, comprising equally spaced side supports 54 projecting therefrom. Hairpin shaped fingers 55 are then mounted by their ends into the grooves cut in the upper and lower sides of supports 54, the fingers being mated to the shape of these supports. The face 57 of the supports 54 is concave and performs, for each hairpin, the function of the rounded portions 15 of the fingers illustrated in Figs. l, 2, 3.

Fig. 6 shows a finger obtained as described above. The delay line obtained is very rigid. The whole structure may have any desired precision degree and has given excellent results.

As for the transmission of ultra high Ifrequency energy, the properties of the delay line according to the invention are the same as those of the interdigital hairpin delay line of known type.

If the ratio of the current traversing the whole length of the line to the total current fed to the input of the line is called focusing ratio, this ratio accounts for the quality of the focusing. Experience shows that for current densities in the beam of 12.5 ma./cm.2 and of 62 ina/cm.2 respectively, this ratio is 47% and 26% for a conventional delay line of the hairpin type, and 75% and 38% with the delay line according to the invention as shown in Fig. 3.

Experience shows also that limited variations of the voltages applied to the angle bars do not substantially modify the focusing ratio and have no effect on the oscillation frequency. In certain particular applications of the tube according to the invention, it is accordingly possible, with a view to simplify the design, to connect electrically angle bar 22 to bar 13 (Fig. 1) and angle bar 20 to the cathode 2, so that no particular voltages are required for the angle bars.

It is to be understood that the invention is vin no way limited to the embodiments shown and described, which have been given only by way of examples.

What we claim is:

1. A travelling wave tube comprising, in a linear vacuum tight enclosure, an electron gun for propagating an electron beam having a rectangular section; an interdigital delay line in coupled relationship to the beam and having two combs each comprising a bar supporting an array of fingers each having two longitudinal legs and two symmetrical transverse portions joining respectively the ends and intermediate points thereof to form a closed contour surrounding the beam; two plane electrodes respectively parallel to, between and each closely adjacent to one of, said bars and insulated therefrom; and terminal connections for bringing said bars to different electrical potentials and each of said plane electrodes to approximately the same potential as the bar remote therefrom.

2. A travelling wave tube comprising, in a linear vacuum tight enclosure, electron emissive, focusing and accelerating means for propagating an electron beam having a rectangular section; an interdigital delay line in coupled relationship to the beam and having two combs each comprising a bar supporting an array of ngers each having two longitudinal legs and two symmetrical transverse portions joining respectively the ends and intermediate points thereof to form a closed contour surrounding the beam; two plane electrodes respectively parallel to, Ibetween and each closely adjacent to one of, said bars and insulated therefrom; terminal connections for bringing each of said electrodes to a potention different from that of the adjacent bar; two grid shaped electrodes placed on either side of and parallel to said delay line fingers; means for bringing said grids to a potential close to that of said emissive means, means for Ibringing said bars to diiferent electrical potentials, and

' means lfor lbringing each of said plane electrodes to apbeing aligned to pass said electron beam, the end of each nger opposite its respective bar being spaced from the opposed other bar, and an auxiliary electrode in each space between the end of each nger and the opposite bar and in proximity to the latter, means for maintaining said bars at different potentials and for maintaining each auxiliary electrode at a potential different than its proximate bar and closer to that of the other bar.

4. A travelling wave tube according to claim 3, further comprising grid-shaped electrodes on opposite sides of said delay line for intercepting secondary emission from said fingers caused by the striking of the electron beam thereon.

5. A travelling \wave tube according to claim 3, wherein each of said fingers comprises hairpin-shaped rod.

References Cited in the file of this patent UNITED STATES PATENTS 2,190,511 Cage Feb. 13, 1940 2,289,756 Clavier July 14, 1942 2,768,322 Fletcher Oct. 23, 1956 2,800,604 Beaver July 23, 1957 2,820,172 Field Ian. 14, 1958 FOREIGN PATENTS 730,687 Great Britain May 25, 1955 

